1. Field of the Invention
The present invention relates to a capacitance detecting circuit and method for detecting a very small capacitance, and also to a fingerprint sensor using the same.
2. Description of the Related Art
As a known fingerprint sensor, which is considered to be most promising in biometrics techniques, the following type of pressure-sensitive capacitance sensor has been developed. Row lines and column lines are formed at predetermined intervals on the surfaces of two films, and the two films are disposed with a predetermined gap therebetween such that they face each other with an insulating film therebetween.
In this pressure-sensitive capacitance sensor, when a finger is placed on the sensor, the shape of the films are changed according to the ridges and valleys of the fingerprint, and accordingly, the intervals between the row lines and the column lines are changed. Thus, the shape of the fingerprint is detected as capacitances at the intersections of the row lines and the column lines.
In this type of sensor, to detect a capacitance of less than several hundred fF (femtofarads), a detecting circuit for converting the capacitance into an electrical signal by using a switched capacitor circuit is conventionally used. More specifically, in this sensor, a sensor capacitor device for detecting the capacitance of a subject by being driven by a first sensor drive signal and a reference capacitor device for generating a reference capacitance for the detecting circuit by being driven by a second sensor drive signal are connected to a common switched capacitor circuit. Then, first and second sample-and-hold circuits, which are alternately operating, sample the output signals of the sensor capacitor device and the reference capacitor device, and then determine the difference between the sampled signals, thereby obtaining a detection signal.
In this detecting circuit, a signal which is proportional to the capacitance Cs of the subject and which is inversely proportional to the feedback capacitance Cf can be stably detected by the common switched capacitor circuit. Additionally, the leakage (feedthrough) of electric charge Qd stored in a parasitic capacitance formed between the gate electrode of a reset switch (feedback control switch) of the switched capacitor circuit and the other electrodes to the these electrodes can be offset. Also, offset components of the reference potential of the switched capacitor circuit or low-frequency noise contained in the input signal can be removed to a certain degree by determining the difference between the sampled signals (for example, see Japanese Unexamined Patent Application Publication No. 8-145717 (paragraphs 0018-0052, FIGS. 1 through 4)).
It is demanded that a capacitance detecting circuit used in, for example, a fingerprint sensor, have high sensitivity since capacitance changes are very small. At the same time, however, the detecting circuit must have resistance to noise (including high-frequency noise) transmitted from a human body or noise from other circuitry.
It is also demanded that the capacitance detecting circuit is not vulnerable to crosstalk noise between adjacent row lines or column lines.
To satisfy these demands, the following type of capacitance detecting circuit can also be considered. At the rise of a column line, a charging voltage corresponding to the electric charge charged in the capacitor at the intersection between the row line and the corresponding column line is detected. Then, at the fall of the column line, a discharging voltage corresponding to the electric charge discharged from the capacitor at the intersection between the row line and the column line is detected. A change in the capacitance is then detected by using the charging voltage and the discharging voltage.
That is, in this capacitor detecting circuit, the difference voltage is determined by subtracting the discharging voltage from the charging voltage so as to detect a change in the capacitance. Accordingly, the voltage offset occurring at the same polarities caused by the feedthrough of an amplifying circuit or offset components generated in other circuits can be eliminated, thereby removing noise having much lower frequencies than the sampling frequency.
In regular detecting circuits including the above-described capacitance detecting circuits, to detect a capacitance change of each sensor device of a capacitance sensor, only a single column line is driven to detect changes in the capacitances Cs at the intersections between the column line and a plurality of row lines. As described above, a capacitance change per sensor device (one intersection) is very small, i.e., about several hundred fF.
Accordingly, in the known capacitor detecting circuits, even if offset components in the circuitry including the amplifying circuit are eliminated, the detecting circuit is influenced by noise originally superposed on the capacitor sensor.
Thus, in the capacitor detecting circuits, conducted noise transmitted to the capacitor sensor via a power supply or a human body is superposed on signals in the row lines and the column lines, thereby making it difficult to precisely detect a capacitance change due to this external disturbance noise.
In inverted fluorescent light, which is mainly used as current fluorescent light, a fluorescent lamp is switched ON by generating high frequencies by using semiconductors, causing noise having a fundamental frequency at a several dozens of KHz level.
In the above-described capacitor detecting circuits, the cycle of the sampling frequency for detecting capacitor changes when determining the difference between the charging voltage and the discharging voltage is close to the cycle of the fundamental frequency of the above-described noise.
Accordingly, in the capacitor detecting circuits, beat components caused by a frequency difference, that is, beat components (beat frequency) equal to the difference between two overlapped waves having very small frequency differences remain, and noise components due to the external disturbance cannot be completely removed.
Thus, when using a fingerprint sensor, if a device including a noise source having a frequency close to the sampling frequency of the capacitor detecting circuit, for example, the above-described inverted fluorescent light, is placed near the user, or if the fingerprint sensor is connected to a device having an inverter circuit used in backlight of a liquid crystal device, external disturbance noise caused by the above-described beat components cannot be completely eliminated. Accordingly, the signal-to-noise (S/N) ratio for detecting capacitance changes is reduced, thereby making it difficult to precisely read the fingerprint of the user.